Fix oh-my-zsh tab completion: drop argcomplete, fix _muse helpers

- Remove argcomplete integration entirely (app.py, pyproject.toml, _completers.py, push/pull/checkout/branch.py) — the oh-my-zsh plugin already owns completion; argcomplete was redundant and non-functional under oh-my-zsh

- _muse_branches: rewrite to read .muse/refs/heads/ directly via find+sed instead of spawning `muse plumbing show-ref | python3` — no subprocess, works even when muse is not on PATH inside the completion subshell

- _muse_remotes: rewrite to read .muse/config.toml directly via grep+sed matching [remotes.<name>] section headers — no subprocess, correct TOML schema (was [remote."name"], actual schema is [remotes.name])

- muse.plugin.zsh §7: remove internal `compinit` call — oh-my-zsh calls compinit after all plugins load; calling it again inside the plugin body breaks completion; just add to fpath and let oh-my-zsh handle the rest